Broken keynav on unlock screen

Affected version

Current master.

Bug summary

Steps to reproduce

1. Type anything to focus the prompt
2. Use any form of keynav:
   • <tab> or <shift><tab>
   • <ctrl><alt>tab to "Unlock Window"

What happened

Key focus is lost: It's impossible to navigate back to the entry in order to unlock, or reset with Esc to start over.

Relevant logs, screenshots, screencasts etc.

Nothing. Even the screen reader remains silent.